1. Field of the Invention
The present invention, in general, relates to furnace apparatus and, in particular, to heat shields for casting furnaces.
2. Description of the Related Art
FIG. 1 is a simplified cross-sectional diagram illustrating a conventional casting furnace 10 (e.g., a directional solidification or single crystal casting furnace). Conventional casting furnace 10 includes a furnace portion 12 disposed above a liquid cooled container 14 (with the locations where a cooling liquid is supplied and a take-out opening provided indicated by labels). Also included in conventional casting furnace 10 is a heat shield 16 located between furnace portion 12 and liquid cooled container 14. Heat shield 16 has a discharge opening 18 therethrough that is aligned with furnace portion 12 and liquid cooled container 14.
During operation of conventional casting furnace 10, a mold 20 holding liquid metal is maintained at an elevated temperature in furnace portion 12. The interior of furnace portion 12 is, therefore, often referred to as the xe2x80x9chot zonexe2x80x9d of conventional casting furnace 10. To affect casting of the liquid metal held in mold 20, mold 20 is lowered from furnace portion 12, through discharge opening 18 and into liquid cooled container 14 (the interior of which is referred to as the xe2x80x9ccool zonexe2x80x9d). Crystal growth in the solidifying liquid metal is controlled by manipulating the temperature of the hot and cold zones and the rate at which mold 20 is lowered from furnace portion 12 into the liquid cooled container 14.
In order to accurately control the crystal growth front in the solidifying liquid metal, a predetermined temperature gradient between the hot zone of the furnace portion and the cool zone of the liquid cooled container is desirable. A drawback of conventional casting furnaces is that the discharge opening in the heat shield allows an undesired transfer of heat between the furnace portion and the liquid cooled container, thus disrupting the temperature gradient. This heat can be transferred, for example, through a gap between the outside of the mold and the heat shield. In other words, a discharge opening that does not closely approximate the contour of the mold can allow undesired heat transfer between the furnace portion and the liquid cooled container. This drawback can be enhanced when the contour (e.g., diameter) of the mold varies across the length (i.e., the vertical axis) of the mold.
To accommodate the use of molds of different contours in a single conventional casting furnace, a given heat shield is customarily removed and replaced with another heat shield that includes a discharge opening of the proper size. Such a heat shield replacement, however, requires that the furnace be shut down and production time lost.
Still needed in the field, therefore, is a heat shield for a casting furnace (e.g., a directional solidification or single crystal casting furnace) that provides for an improved control of the temperature gradient between the hot zone of the furnace portion and the cool zone of the liquid cooled container and, thus, improved control of the crystal growth front. In addition, the heat shield should accommodate molds of different and varying contours.
The present invention provides a heat shield for a casting furnace (e.g., a directional solidification or single crystal casting furnace) with improved control of a temperature gradient between the hot zone of the furnace portion and the cool zone of the liquid cooled container, thereby improving control of crystal growth. In addition, the heat shield easily accommodates molds of different and varying contours without having to shut down the furnace and lose production time.
A heat shield according to one exemplary embodiment of the present invention is configured for placement between a furnace portion and a liquid cooled container of a casting furnace (e.g., a directional solidification or single crystal casting furnace) and includes a plurality of heat insulating plates, each with a leading edge. These heat insulating plates are arranged such that at least a portion of their leading edges defines a discharge opening circumscribed (i.e., surrounded) by the heat insulating plates. The plurality of heat insulating plates are moveable in a manner that adjusts (i.e., increases or decreases) the size of the discharge opening.
The heat shield also includes a rotatable disk operatively coupled to the heat insulating plates such that when the rotatable disk is rotated in one direction, the heat insulating plates are moved in a manner which decreases the size of the discharge opening. Furthermore, when the rotatable disk is rotated in another direction, the heat insulating plates are moved in a manner which increases the size of the discharge opening.
Since the discharge opening of heat shields according to one exemplary embodiment of the present invention can be easily adjusted (i.e., the size of the discharge opening can be increased or decreased) during operation of the furnace to follow the contour of a mold, a gap between the outside of a mold and the heat shield can be precisely controlled. For example, such a gap can be controlled to a minimum size, thereby eliminating as much heat transfer through the gap as possible and providing a relatively sharp temperature gradient between a hot zone of the furnace portion and a cool zone of the liquid cooled container.